Executives with the firms confirmed they are in negotiations with park owner Equity Office Properties, but declined to comment further.

The town of Burlington has approved a major expansion of the office campus that would allow its owners to build up to 450,000 square feet of additional commercial space, including stores, restaurants, office buildings, a hotel, and parking.

Tavern in the Square recently opened a restaurant in the park, and another eatery is being planned.

An Equity Office Properties spokeswoman said executives were unavailable for comment Thursday.

The firm’s parent, Blackstone Group, has been weighing the sale of properties within it US office portfolio, which includes many major buildings in Boston and surrounding communities.

If finalized, the sale of New England Executive Park would be among only a few major suburban properties to change hands in recent years.

Boston Properties purchased the Bay Colony Corporate Center in Waltham in 2011, and is renovating its buildings and adding new cafes and other amenities.

With many companies interested in moving into urban centers, real estate investors have shied away from buying large suburban properties that are perceived as being more difficult to keep filled.

But some also believe suburban campuses offer a viable alternative to higher-priced city complexes, especially if they can be outfitted with restaurants, bars, shopping, and other amenities that make them more lively places to work.

The New England Executive Park, located on Mall Road between the Burlington Mall and the Lahey Clinic, includes 10 buildings over 56 acres.

Its 1 million square feet of office space is 86 percent occupied by a range of companies, including defense contractor BAE Systems, TD Bank, and Charles River Systems Inc., a software maker.

Charles River Realty and National Development could move forward with the office park’s expansion as early as this year.

National Development has considerable experience with such projects in communities across the region.

The company built a world headquarters complex for ­Staples Inc. in Framingham as well as the Station Landing ­residential and retail complex in Medford. It is currently redeveloping the Boston Herald’s former South End headquarters into a mixed-use complex with apartments and a Whole Foods grocery store.